1. Field of the Invention
The present invention relates generally to the fields of organic chemistry and optically anisotropic coatings. More specifically, the present invention relates to lyotropic chromophoric compounds, lyotropic liquid crystal systems comprising one or more lyotropic chromophoric compounds, and optically isotropic or anisotropic films.
2. Description of the Related Art
Optical elements are increasingly based on new materials possessing specific, precisely controllable properties. An important element in many modern visual display systems is an optically anisotropic film having a combination of optical and other characteristics that can be adjusted to suit the requirements of a particular device, since each device often has its own set of requirements.
The increased popularity of liquid crystal displays (LCDs) has motivated studies of various liquid crystal (LC) compounds. Earlier researchers focused on thermotropic LC compounds that could be oriented into anisotropic films by mechanical forces. However, the forced orientation of the molecules in a thermotropic LC film would tend to disappear when the forces were discontinued. On the other hand, lyotropic liquid crystal (LLC) films are capable of retaining their dichroic orientation after the mechanical force is removed. Suitable materials include those that are capable of forming LC mesophases that can be oriented to form an anisotropic film.
Various polymeric materials have been used in the manufacture of optically anisotropic films. Films based on such materials may acquire anisotropic optical properties through uniaxial extension and modification with organic dyes or iodine. In many applications, the base polymer is polyvinyl alcohol (PVA). Such films are described in greater detail in the monograph Liquid Crystals: Applications and Uses, B. Bahadur (ed.), World Scientific, Singapore—N.Y. (1990), Vol. 1, p. 101. However, the low thermal stability of PVA-based films can limit their application. Development of new materials and methods for the synthesis of optically anisotropic films possessing improved characteristics is therefore quite advantageous. Particularly, films having properties such as higher heat resistance, convenient synthesis, and uniformity are highly desirable.
In recent years, there has been increasing demand for films possessing high optical anisotropy that are also characterized by improved selectivity in various wavelength ranges. Films having absorption maxima at different locations in the wide spectral range from the infrared (IR) to the ultraviolet (UV) are very desirable. Organic dichroic molecules are known to pack into supramolecular complexes that are generally shaped like columns. These columns form the basic structural units of a mesophase, and the mesophases can be oriented to form an anisotropic film with strong dichroism. Anisotropic materials have been synthesized based on water soluble organic dyes, for examples, in U.S. Pat. Nos. 5,739,296 and 6,174,394 and European patent EP 0961138. These materials exhibit high absorbance in the visible spectral region. While they may be advantageous for many applications, the absorbance profiles of these compounds limit their application in forming transparent double refraction films.
Additionally, currently available film application technologies typically require that the process parameters, for examples, dye concentration, film formation temperature, etc., be thoroughly selected and strictly followed during the formation of the films. However, even if all the conditions of film formation are precisely controlled, random local variation of the coating regime may still occur due to the formation of misorientation zones and/or microdefects. This may be a result of non-uniform micro- and macrocrystallization processes in the course of solvent removal upon applying the LLC system (e.g., LLC solution) onto a substrate surface. In addition, the probability of forming a coating with non-uniform thickness using the currently available dyes remains high, which in turn decreases the reproducibility of the target film parameters.
Anisotropic films that are selective in different wavelength ranges are required by growing number of new applications. It is therefore desirable to develop new varieties of compounds capable of forming an LLC phase and films with the required properties. Films with different absorbance maxima location in wide spectral range from the infrared to the ultraviolet are also desirable. However, only a small number of currently available dyes are useful in the formation of lyotropic mesophase. Thus, new LC dyes are now an object of attention.
Optically anisotropic films may be formed on glass, plastic, or other substrate materials. Films which exhibit high quality optical characteristics may be used as polarizers, which are described in Bobrov, et al., Environmental and Optical Testing of Optiva Thin Crystal Film® Polarizers, Proceedings of the 10th SID Symposium “Advanced display technologies,” (Minsk, Republic of Belarus, Sep. 18-21, 2001), p. 23 to 30. Methods for the preparation of such films, including those with a high degree of crystallinity, are described in PCT Publication No. WO 02/063,660. The aforementioned PTCA derivatives are capable of forming LLC phases, and anisotropic films obtained using the LLC system possess excellent optical characteristics and exhibit good performance as polarizers.
Naphthalene- and perylene-tetracarboxyl bisbenzoimidazole disulfoderivatives are dichroic dyes capable of forming LLC systems that are also useful for the preparation of optical anisotropic films. Both naphthalene- and perylene-tetracarboxyl bisbenzoimidazole are insoluble in water, but may be converted to water-soluble form through a sulfonation process. To produce the disulfoderivative, an effective amount of naphthalene- or perylene-tetracarboxyl bisbenzoimidazole is added to oleum under prescribed conditions.
One of the main disadvantages of the previously described water-soluble naphthalene- and perylene-tetracarboxyl bisbenzoimidazole disulfoderivatives is the complexity of producing anisotropic films with uniform properties over the substrate surface. The complexity results from their phase instability, the likelihood of forming disorientation zones and micro- and macro-crystallization during solvent removal after the liquid crystal is coated on a substrate surface. These drawbacks complicated the process of forming films with high optical characteristics. Poor reproducibility necessitates accurate adjusting and strict control of fixed technological conditions at each film forming stage from coating to drying which may dramatically increase film production expense.